2011-02-14_PERMIT FILE - C1980007A (3)

Home Browse Search

Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 14 • • Panels El to E8: For these eight panels, projected horizontal tensile strain ranges from 0.004 to 0.022 (0.4 to 2.2 percent) where the planned coal extraction thickness is 8 feet; from 0.007 to 0.027 (0.7 to 2.7 percent) where the extraction thickness equals 12 feet; and from 0.007 to 0.039 (0.7 to 3.9 percent) where the extraction thickness equals 14 feet. Horizontal compressive strain ranges from -0.004 to -0.024 (44 to -2.4 percent) where the extraction thickness equals 8 feet; -0.007 to -0.029 (47 to -2.9 percent) where the thickness equals 12 feet; and -0.007 to -0.042 (47 to -4.2 percent) where it equals 14 feet (Table 2). • Panel E9: Predicted horizontal tensile strain in this single panel ranges from 0.018 to 0.027 (1.8 to 2.7 percent) where the coal extraction thickness equals 12 feet and 0.021 to 0.031 (2.1 to 3.1 percent) where the extraction thickness equals 14 feet. Compressive strain this panel is predicted to range from -0.018 to -0.029 (4.8 to -2.9 percent) where the extraction thickness equals 12 feet and -0.021 to -0.034 ( -2.1 to -3.4 percent) where it equals 14 feet (Table 2). Da Fork Mining Area: Maximum tensile and compressive horizontal strains are calculated, using the values obtained from the West Elk Mine area, and as projected in Figure 5 (Table 2). These values are believed to be conservative, based on Dunrud's annual observations over the last seven years in the Apache Rocks mining area. • Panels E2 to E6: For the five panels that extend into the Dry Fork mining area, projected horizontal tensile strain ranges from 0.007159 to 0.0085 (0.7 to 0.9 percent) where the planned coal extraction thickness is 9 feet, and from 0.0040 to 0.014 (0.6 to 1.4 percent) where the extraction thickness equals 10 feet. Horizontal compressive strain ranges from - 0.0071 to - 0.0092 (47 to -0.9 percent) where the extraction thickness equals 9 feet, and - 0.0040 to -0.015 (44 to -1.5 percent) where the thickness equals 10 feet (Table 3). 5.3.2 Maximum Projected Depths of Surface Cracks Curvature, or differential tilt (curvature is the second derivative of vertical displacement with respect to horizontal distance) of subsided rock layers causes horizontal strain. Comparison of calculated curvature values and horizontal tensile strain derived from horizontal displacement measurements, therefore, provides a means of calculating the depth of the neutral surface, and hence the maximum depth of tension cracks from the surface. The neutral surface is the boundary between tensile and compressive strain In terrains with slopes less than about 30 percent, the depth of the neutral surface can be estimated by dividing the maximum horizontal strain values by those of maximum curvature at a given location. The calculated depth of the tension zone to the neutral surface -the boundary between tension above and compression below -ranges from 50 to 100 feet in the subsidence 831 - 032.810 Wright Water Engineers, Inc.